CD4+ T cell recognition of cognate MHC class II:peptide complexes on antigen presenting cells (APC) triggers large-scale molecular rearrangements at the T-APC interface forming a structure called the immune synapse. At the synapse, T cells capture large membrane fragments and associated proteins from the APC in a process termed trogocytosis. While this phenomenon has the potential to significantly alter the biology of the individual T cell, we currently have only limited understanding of the biological consequences of this process. As a result of trogocytosis, CD4+ T cells display immunological synapse components on their surface including cognate MHC:peptide and costimulatory molecules, such as CD80. We have previously shown that these molecules mediate intracellular signaling within the T cell, presumably by engaging their receptors on the cell. In this proposal we will examine the impact that this signaling has on the individual T cell. Our preliminary data suggests that this trogocytosis-associated signaling maintains effector cytokine (IL-4) production and may mediate the conversion of T cells to a TH2 phenotype in vitro. The central hypothesis of this proposal is that is that trogocytosis-mediated, sustained TCR signaling sustains effector cytokine production and mediates the conversion of trog+ T cells to a TH2 phenotype. This hypothesis will be examined using 2 specific aims: Aim #1) Determine whether the the observed increase in the frequency of trog+ IL-4+ and GATA-3hi cells is the result of signaling from the trogocytosed molecules. Aim #2) Determine whether the observed increase in IL-4 and GATA-3 expression by trog+ cells is reflective of an intrinsic difference in the ability of TH1 and TH2 to perform trogocytosis or the result of selective survival and/or conversion to TH2. Using flow cytometry, qRT-PCR and 3D wide-field deconvolution microscopy, these experiments will involve both in vitro and in vivo approaches to monitor effector cytokine production as a result of cell- autonomous signaling and examination of the phenotype of the cells that have acquired APC membrane fragments via trogocytosis. The results from this proposal will provide important insight into the role of trogycytosis in T cell effector function and effector phenotype and will lad to additional lines of inquiry to elucidate the in vivo functions of trogocytosis.